Solid oxide fuel cells (SOFC) offer power generation in designs that produce energy cleanly and efficiently, and as the technology marches towards maturity the wide-spread use of fuel cells becomes closer. Inventions such as the standard package design by Gilleft, U.S. Pat. No. 6,764,784 have help this advancement.
An example of a typical tubular SOFC design is illustrated in cross-section in FIG. 1 with an exaggerated scale. Air flows along the inside of the support tube 2, which has layers on it first a cathode or air electrode substrate 4, then a thin solid electrolyte layer 6, and finally an anode or fuel electrode 8. Finally, there is an interconnection 10 that received electron flow from adjoining fuel cells. Hydrocarbon fuels flowing along the outside of the cell mixes with oxygen delivered by the thin solid electrolyte layer 6 from the air, to form water, carbon dioxide and electrons.
FIG. 2 illustrates a scaled cross-sectional close-up of the anode layers. Oxygen transports through the air electrode substrate 4 and the electrolyte 6 to mix with the hydrocarbon fuels in the fuel electrode 8 to produce water, carbon dioxide and electrons.
The composition of the various anode layers necessarily needs to be different from one another. The electrolyte layer 6 is composed of Y2O3 or Sc2O3 doped ZrO2, or Sm-doped CeO2 or an equivalent (collectively referred to herein as YSZ/ScSZ) where Y and Sc are rare earth elements. The air electrode substrate 4, is porous, about 30% porosity, and a homogeneous mix of Ca and Ce doped LaMnO3. The fuel electrode 8 is also porous, about 30% porosity, and is a homogeneous mix of about 25 wt % YSZ/ScSZ and 75 wt % conductive metal.
However, the layers of the fuel electrode perform multiple functions which can be at odds with each other. For example, in the fuel electrode layer, the YSZ/ScSZ content helps the fuel oxidation reaction, but hinders the conduction of the electron flow. Therefore, the prior art has sought to maximize the net benefit of reaction versus conduction in the percentages given above, which, of course involves a compromise on each aspect.
The fuel electrode layers are deposited onto the support tube by slurry-coating techniques such as screen-printing and dip-coating, which give the homogenous layers discussed above.
What is needed is a method and apparatus that will produce fuel cells with layers that have a facile fuel oxidation reaction, as well as a better electronic conduction.
Other difficulties with the prior art also exist, some of which will be apparent upon further reading.